The present invention is a mixed oxide solid solution containing a tetravalent and a pentavalent cation that can be used as a support for a metal combustion catalyst. The invention is furthermore a combustion catalyst containing the mixed oxide solid solution and a method of making the mixed oxide solid solution.
Combustion processes oftentimes produce a variety of environmental pollutants. The pollutants may be created due to the presence of a contaminant in a fuel source or due to an imperfect combustion process. In either event, the treatment of combustion products with certain types of catalytic combustion technologies can substantially reduce the entry of combustion created pollutants into the environment.
One type of catalytic combustion technology is the XONON(copyright) catalyst unit, which is a proprietary product of Catalytica Combustion Systems, Inc. This catalyst unit possesses a number of advantages over other combustion catalyst systems. For instance, its provides for improved temperature control throughout the catalyst structure: both the temperature rise along the length of the structure and the maximum temperature at the catalytic wall are limited. This control allows for an increased lifetime of highly active catalysts that are typically deposited on the walls of a monolithic structure.
The XONON(copyright) unit consists of a corrugated metal foil that has been coated with a catalytic layer. The coated foil is wound into a spiral, forming a cylindrical structure with longitudinal passages. The unit configuration, and related variations, are described in a number of U.S. patents, including U.S. Pat. Nos. 5,232,357, 5,259,754, 5,405,260 and 5,518,697.
The catalytic layer coated on the corrugated metal foil typically consists of a catalyst supported by a variety of refractory oxides. Such oxides include alumina, silica, titania and magnesia that may or may not have been doped with a stabilizing agent. U.S. Pat. No. 5,5259,754 (""754 patent) reports the use of zirconia as a support for certain metal catalysts, noting that it is a preferred support for palladium catalysts.
Zirconia is less reactive with transition metals than alumina, rare earth oxides, alkaline earth oxides, titania or silica. This is important since transition metals are the active catalytic component in a variety of combustion processes. A reaction between the catalyst and the support deactivates the catalyst, thereby decreasing the efficiency of the catalyst unit.
Despite the advantage offered by zirconia, it is not a commercially acceptable catalyst support material. Catalytic combustion processes produce water vapor and are performed at very high temperatures. Zirconia rapidly sinters in the presence of water vapor at high temperatures. The sintering decreases the surface area of the zirconia, making it substantially less effective as a support for the catalytic component.
Zirconia can be stabilized to sintering and coarsening through the addition of silica oxide. The silica greatly extends the performance and useful operating range of the zirconium based materials. The use of zirconia and silica-zirconia mixed oxides as combustion catalyst supports is discussed in the following U.S. patents: U.S. Pat. No. 5,183,401, U.S. Pat. No. 5,232,357, U.S. Pat. No. 5,248,349, U.S. Pat. No. 5,259,754 and U.S. Pat. No. 5,405,260.
Several studies have been performed to determine the effect of other additives on the thermal properties of zirconium oxide. Mercera et al. reports that the addition of MgO to zirconium oxide slightly improved its thermal stability at 500xc2x0 C. Applied Catalysis (1991) 71, 363-391. It was further reported that the addition of CaO, Y2O3 and La2O3 increases the stability of zirconium oxide up to 700xc2x0 C., but that the surface area of the mixed oxides decreases rapidly above 800xc2x0 C. Id. Turlier et al. similarly discusses the addition of lanthanum and yttrium to zirconia in an attempt to increase its thermal stability. Applied Catalysis (1987) 29, 305-310.
There are also articles that report the addition of tantalum to zirconium oxide. Pissenberger and Gritzner discuss the preparation of tantalum and niobium doped zirconium oxide. J. Materials Science Letters (1995) 14, 1580. Gritzner and Puchner report the preparation of vanadium, niobium and tantalum doped zirconia ceramics. J. European Ceramic Society (1994) 13, 387-394. The mixed oxides were prepared through the hydrolysis of the corresponding alkoxides and have lower sintering temperatures than zirconium oxide.
There is a need for a metal catalyst support that is stable at high temperatures in the presence of water vapor. Although there are reports of zirconia mixed oxides, none describe the preparation of stable zirconium oxide materials with a surface area above 10 m2/g at temperatures greater than 1000xc2x0 C. in air plus high concentrations of water vapor. Furthermore, none of the reports describes the preparation of such zirconia mixed oxides starting from molecularly mixed materials.
The present invention is a mixed oxide solid solution, a process for making the mixed oxide solid solution and a combustion catalyst containing the mixed oxide solid solution.
The mixed oxide solid solution includes a tetravalent cation and a pentavalent cation. The tetravalent cation is zirconium(+4), hafnlium(+4) or thorium(+4). Preferably, it is zirconium (+4). The pentavalent cation is preferably tantalum(+5), niobium(+5) or bismuth(+5). More preferably, the tetravalent cation is tantalum (+5).
The mixed oxide solid solution contains from about 2 mole percent to about 70 mole percent of the pentavalent cation. Preferably, it contains from about 10 mole percent to about 50 mole percent of the pentavalent cation. More preferably, the solid solution contains from about 15 mole percent to about 35 mole percent of the pentavalent cation.
The mixed oxide solid solution has a relatively high surface area at high temperatures in the presence of water vapor. At 1000xc2x0 C. in air and about 10% water vapor, for example, the solid solution has a surface area of greater than about 10 m2/g. Preferably, the solid solution has a surface area of greater than about 15 m2/g, when heated at 1000xc2x0 C. in air and about 10% water vapor.
The process for making the mixed oxide solid solution includes the following steps: (1) forming a molecularly uniform hydrous oxide gel; (2) drying the gel under supercritical conditions; and, (3) calcining the dried gel. Preferably, the gel formation step includes the steps of (1) mixing a tetravalent cation alkoxide and a pentavalent cation oxide to form a mixture, and (2) adding a solution comprising an acid, an alcohol and water to the mixture. The supercritical drying step preferably includes the steps of (1) placing the gel in a reaction chamber, (2) adding a solvent to the gel to form a mixture, and (3) removing the solvent from the mixture above its critical temperature.
The combustion catalyst is a monolithic catalytic combustion catalyst. It includes a metal support, a mixed oxide solid solution support and a catalytically active component coated on the mixed oxide support. The mixed oxide solid solution support contains a tetravalent and a pentavalent cation.